1. Field of the Invention
The invention relates to a wheel brake apparatus and a method for operating the wheel brake apparatus including an electric motor and a rotation/translation conversion mechanism.
2. Description of the Prior Art
A wheel brake apparatus of the type with which this invention is concerned is disclosed by DE 195 29 664 A1. The known wheel brake apparatus has an electric motor which can drive a rotation/translation conversion mechanism. The rotation/translation conversion mechanism of the known wheel brake apparatus is embodied as a helical gear. The rotation/translation conversion mechanism can move a hydraulically acting working piston in a working cylinder. The working cylinder communicates with a cylinder in which a brake lining piston is movably contained. The movement of the working piston in the working cylinder moves the brake lining piston in its cylinder and because the cylinder diameters are different, a hydraulic path reduction and force increase can be produced. The brake lining piston can press a frictional brake lining against a brake body non-rotatably connected to a vehicle wheel, for example a brake disk or brake drum, in order to produce a brake force or a brake moment. The known wheel brake apparatus combines an electromechanical drive mechanism with a hydraulic drive mechanism.
In the wheel brake apparatus according to the invention, the rotation/translation conversion mechanism can be brought into a mechanical connection with the brake lining piston and as a result, the brake lining piston can be moved. This has the advantage that a parking brake function of the wheel brake apparatus can be produced which is independent of the hydraulics. A brake force built up in the parking brake function remains unchanged for a long time since the brake force is built up in an exclusively mechanical fashion and as a result, leakage losses in the hydraulics are prevented from reducing the brake force. Another advantage of the wheel brake apparatus according to the invention is its ability to be actuated in an exclusively mechanical fashion, for example in the event of a leak in the hydraulics of the wheel brake apparatus. This permits a mechanical emergency braking operation in the event of a malfunction in the hydraulics. In comparison to an exclusively electromechanical wheel brake apparatus, the wheel brake apparatus according to the invention has the advantage that it can easily be embodied with two or more brake lining pistons and can thus be inexpensively designed, for example in the form of a fixed yoke brake device.
According to one embodiment, a movable element of the rotation/translation conversion mechanism can be moved into contact with the brake lining piston and in this manner, the brake lining piston can be mechanically moved in order to press the frictional brake lining against the brake body. The movable element can be a spindle of a rotation/translation conversion mechanism embodied as a helical gear.
Another possibility for mechanically moving the brake lining piston is to provide the working piston so that it can be moved into contact with the brake lining piston.
In a preferred embodiment of the invention, the wheel brake apparatus has a valve that can be opened and closed, which is connected to the working cylinder and to the cylinder of the brake lining piston. When the valve is open, a hydraulic operational connection between the working piston and the brake lining piston is disengaged and the brake lining piston can be moved mechanically with the rotation/translation conversion mechanism. Moreover, opening the valve makes possible for the wheel brake apparatus to be released in the event of a malfunction in its electromechanical drive mechanism. The valve is preferably open in a normal position.
According to one modification, the wheel brake apparatus of the invention has a pressure sensor for measuring the hydraulic pressure. The pressure sensor can be used, for example, to determine a brake force of the wheel brake apparatus since the brake force is at least approximately proportional to the hydraulic pressure.
According to a further embodiment, the wheel brake apparatus according to the invention has a rotation angle sensor for the rotor of the electric motor or the rotation/translation conversion mechanism. The rotation angle sensor can measure a rotation angle of the rotor of the electric motor or of a rotating part of the rotation/translation conversion mechanism, in complete rotations and/or in fractions of a rotation. Since the rotation angle is proportional to a displacement path of the rotation/translation conversion mechanism, a displacement path of the rotation/translation conversion mechanism can therefore be determined. A displacement path sensor can also be added.
The correlation of the rotation angle measured with the rotation angle sensor and the hydraulic pressure measured with the pressure sensor permits the proper functioning of the wheel brake apparatus to be monitored. When the wheel brake apparatus is functioning properly, these two values maintain a particular proportion to each other in every operating state of the wheel brake apparatus. During operation of the wheel brake apparatus according to the invention, if the proportion of the two values to each other diverges significantly from their proportion during proper operation wheel brake apparatus, then this indicates a malfunction.
Instead of, or in addition to the rotation angle sensor, the wheel brake apparatus can also have a path sensor for the displacement path of the rotation/translation conversion mechanism.
According to another modification, the rotation/translation conversion mechanism of the wheel brake apparatus according to the invention is embodied as self locking-free so that when the electric motor is without current, a pressing force of the frictional brake lining against the brake body decreases to a negligible value. As a result, it is possible for the wheel brake apparatus to be released in the event of a failure of the power supply of the electric motor.
For the parking brake function, in order to maintain a brake force without current once the brake force has been exerted, a brake is provided with which the rotor of the electric motor or the rotation/translation conversion mechanism can be locked in place.
The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawing, which shows a partially simplified schematic representation of an axial section through a wheel brake apparatus according to the invention.